The present invention relates to a presensitized plate for use in making a lithographic printing plate (hereinafter referred to as "PS plate") comprising an aluminum substrate and a developing process therefor and more specifically to a PS plate suitably processed by an automatic developing machine as well as a developing method thereof.
A positive-working PS plate which has been widely used comprises an aluminum plate as a substrate provided thereon with a light-sensitive layer comprising an o-quinonediazide compound. The o-quinonediazide compound is known to be converted into a carboxylic acid through exposure to ultraviolet rays. Accordingly, when a PS plate of this type which is exposed to ultraviolet rays is developed with an aqueous alkali solution, only the exposed portion of the light-sensitive layer comprising the compound is removed and the corresponding surface of the substrate is exposed. The surface of the substrate exposed through development (non-image area) receives water and repels an oil-based ink because of the hydrophilic properties of the exposed surface of the substrate, while the light-sensitive layer (image area) which is not removed through development repels water and accepts an oil-based ink because of the lipophilic properties of the light-sensitive layer. There have been known various aqueous alkaline solutions used as developers for the positive-working PS plates of this type, but most preferred are aqueous solutions of silicates such as sodium silicate and potassium silicate. This is because the developing ability of the developer may be controlled to some extent by adjusting the ratio of the silicon oxide SiO.sub.2 to the alkali metal oxide M.sub.2 O as the components of the silicate (in general expressed in terms of molar ratio: [SiO.sub.2 ]/[M.sub.2 O]) and the concentrations thereof.
These silicates have been preferably used as components of developers not only for the aforementioned positive-working PS plates, but also for negative-working PS plates for reversal processing which comprise o-quinonediazide-containing light-sensitive layers as disclosed in Japanese Patent Publication for Opposition Purpose (hereinafter referred to as "J.P. KOKOKU") No. Sho 56-14970 and U.S. Pat. No. 4,576,901, negative-working PS plates which comprise alkali-soluble diazonium salts in the light-sensitive layers and negative-working PS plates which comprise light-sensitive layers containing, as photocrosslinkable agents, resins carrying dimethylmaleimido groups on the side chains as described in European Laid-Open Patent No. 0492959A.
Recently, there have been widely used automatic machines for developing PS plates in the fields of plate-making and printing to rationalize and standardize the plate-making operations. The automatic developing machine generally comprises a device for conveying a PS plate to be processed, a tank for accommodating a developer and a device for spraying the developer on the PS plate so that the PS plate is developed by spraying the pumped up developer on the PS plate through a spray nozzle while horizontally conveying the PS plate. Alternatively, there has recently been known a developing method comprising dipping a PS plate in a bath for development while conveying the PS plate by means of, for instance, dipped rolls for guiding the plate in the developer.
Japanese Un-examined Patent Publication (hereinafter referred to as "J.P. KOKAI") No. Sho 54-62004 (=U.S. Pat. No. 4,259,434) discloses that, when a positive-working PS plate is developed with such an automatic developing machine, a large quantity of positive-working PS plates can be processed over a long time without exchanging the developer accommodated in a tank through the use of an aqueous solution of sodium silicate having a molar ratio: SiO.sub.2 /Na.sub.2 O ranging from 1.0 to 1.5 (i.e., [SiO.sub.2 ]/[Na.sub.2 O] of 1.0 to 1.5) and an SiO.sub.2 content ranging from 1 to 4% by weight, as a developer, and by continuously or intermittently adding, to the developer, an aqueous solution (replenisher) of sodium silicate having a molar ratio: SiO.sub.2 /Na.sub.2 O ranging from 0.5 to 1.5 (i.e., [SiO.sub.2 ]/[Na.sub.2 O] of 0.5 to 1.5).
If a substantially large quantity of PS plates are processed, however, this developing method suffers from a problem in that insoluble matter is formed in the developer. This in turn leads to the adhesion thereof to the resulting lithographic printing plates and the clogging of a spray nozzle and a filter. It has been known that the formation of insoluble matter becomes conspicuous in the development of the positive-working PS plate comprising an aluminum plate having an anodized layer as a substrate. U.S. Pat. No. 4,259,434 proposes a technique for eliminating the drawback in which the developer used comprises a solution of an alkali metal silicate having a molar ratio [SiO.sub.2 ]/[M] ranging from 0.5 to 0.75 (i.e., [SiO.sub.2 ]/[M.sub.2 O] of 1.0 to 1.5) and an SiO.sub.2 content of 1 to 4% by weight; the replenisher used comprises a solution of an alkali metal silicate having a molar ratio [SiO.sub.2 ]/[M] ranging from 0.25 to 0.75 (i.e., [SiO.sub.2 ]/[M.sub.2 O] of 0.5 to 1.5) and both developer and replenisher contain at least 20% of potassium on the basis of the total gram atoms of the alkali metals present therein. This method allows the elimination of the problem concerning the formation of insoluble matter in the developer, but suffers from other problems in that the activity of the replenisher is rather insufficient and that the method requires the use of a large amount of the replenisher. These drawbacks can be eliminated by the method disclosed in European Laid-Open Patent No. 0347245A in which the development is performed in an automatic developing machine comprising a dip-developing zone provided with a floating cover for preventing any contact between the air and the surface of the developer, but it has still been requested to further reduce the running cost and the amount of the waste liquor. In particular, the reduction in the industrial waste has been attracted special interest recently on a worldwide scale.
European Laid-Open Patent No. 0490515A elucidates that the anodized layer on the face (hereinafter referred to as "back face of substrate") of an aluminum substrate opposite to that carrying a light-sensitive layer is dissolved in a developer to a large extent during developing a PS plate and that this becomes a cause of the formation of insoluble matter in the developer. This patent further discloses an effective means for solving this problem, which comprises applying a coating layer (protective layer) consisting of an organic polymeric compound onto the back face of the aluminum substrate.
However, some of these protective layers used get swollen during printing operations depending on the kinds of agents used, this in turn results in a change of the printing pressure and accordingly deteriorates the printing durability of the resulting lithographic printing plate. Moreover, it has also been found that the application of a hydrophobic organic polymer layer onto the back face suffers from a problem of background contamination of copies due to adhesion of a lipophilic substance such as an ink to the back face during using the printing plate.
On the other hand, J.P. KOKAI No. Hei 3-90388 discloses a method for inhibiting the deposit of white powder on the back face of a PS plate due to the action of a developer, which comprises treating the back face of the PS plate with an alkali metal silicate. However, this method suffers from various problems. For instance, the method is complicated in that it requires the use of a processing solution maintained at a high temperature ranging from 50.degree. to 90.degree. C. and an additional treatment with an alkali after the processing. Moreover, the processing solution is a strong alkaline aqueous solution and accordingly, is insufficient in an uniformly spreading ability and would reduce the adhesion between the light-sensitive layer and the aluminum substrate if the processing solution accidentally passes around behind the light-sensitive layer side. In addition, the back face treated according to this method is hydrophilic. Therefore, if a plurality of the sheet-like PS plates of this type are put in a stack, this leads to the adhesion of a hydrophilic mat layer formed on the light-sensitive layer to the back face and in turn results in the adhesion between neighbouring two PS plates and peeling off of some light-sensitive films.